Capacitors with high volumetric energy density, high operating temperature, low equivalent series resistance (ESR), and long lifetime are critical components for pulse-power, automotive, and industrial electronics. The physical characteristics of the dielectric material in the capacitor are the primary determining factors for the performance of a capacitor. Accordingly, improvements in one or more of the physical properties of the dielectric material in a capacitor can result in corresponding performance improvements in the capacitor component, usually resulting in performance and lifetime enhancements of the electronics system or product in which it is embedded. Since improvements in capacitor dielectric can directly influence product size, product reliability, and product efficiency, there is a high value associated with such improvements.
Certain improvements in capacitor dielectric materials can be considered as enabling to a particular technology application. For example, capacitors with high permittivity, high dielectric strength, low ESR, and low dielectric dissipation factor will allow high frequency or pulse-power applications to be reduced to a practical size. High temperature operation will greatly simplify next-generation electric vehicles. Improved dielectrics will enable the specific power and reliability of switching power supplies, power conditioners, and filters to be increased. Improved energy density will decrease the area presently devoted to capacitor devices on printed circuit boards, reducing the weight and size of power conditioning systems, power supplies and down-hole tools for use in oil or gas wells.
To reduce the size of a capacitor while retaining all other physical and electrical characteristics, either an increase in the capacitor dielectric constant or dielectric breakdown strength is necessary. Both are fulfilled with the development of new thin, flexible dielectrics having high voltage breakdown strength, a high dielectric constant and a low ESR loss. Some applications additionally require a stable dielectric constant with no reduction in lifetime at temperatures exceeding 150[deg.] C.
High voltage non-polar capacitors are conventionally made using a metalized polymer film that is wound into a cylindrical shape. In conventional wound capacitors, the dielectric material is typically a polymer film. Common polymer dielectric materials include polycarbonate, polyethylene terephthalate (PET, also known as polyester), polypropylene, polystyrene, and polysulfone. Polymer dielectric-based foil capacitors are generally fabricated by placing alternating sheets of polymer and metal foil in a stack and rolling the stack into a tubular shape or depositing a metal film on one side of the polymer then rolling two stacked metalized polymer films into a tubular shape. Electrical wires are connected to each metal foil. The dielectric material exists in the form of self-supporting layers that are thick enough to sustain the necessary operating voltage (typically at least 3-6 micrometers). Unfortunately, the large thickness of the polymer sheets reduces the energy storage density. Usually the dielectric constant of these capacitors changes and the lifetime is shortened at temperatures in excess of 100-150° C. due to deficiencies in the polymer material. Alternately, two polymer films coated with a thin layer of metal (usually 17-100 nanometers thick) are wound into a tubular shape to form a capacitor. The thin metal film has the advantage of clearing any short that may form if the polymer dielectric breaks down during operation. This may extend the life of the capacitor and minimize the chances of catastrophic failure of the capacitor. Conventional film capacitors do not have high energy density because the relative permittivity (also known as dielectric constant κ) of the film is relatively low, e.g., less than about 5.
Amorphous SiO2, HfO2, other metal oxides and stacks of amorphous oxides and nitrides, e.g. SiO2/Si3N4, are disclosed in prior art as dielectric materials of capacitors. A flexible substrate comprised of an insulating polymer film coated with thin metal layers on both sides of the film and a process to deposit the amorphous oxides and oxide/nitride layers on the film to produce a material that can be rolled into cylindrical shapes is also disclosed in prior art.
In the prior art the metallized film capacitors are known. These capacitors include two tightly wound sheets, wrapped around a core. Each sheet includes a dielectric layer and a metallized layer. The metallized layer does not extend to the opposing ends of the sheet leaving a non-metallized margin on opposing sides of each sheet. The ends of the roll formed from the two tightly wound sheets are sprayed with a conductive metal to form a conducting termination for the capacitor. Capacitors made in this way can be used for a variety of purposes depending upon factors such as the type of sheet material as well as the thickness and dielectric constant of the sheet. Typical materials for the sheet are, for example, oriented polypropylene or poly-(ethylene)-terephtalate. The conductive metal termination is typically applied in a vacuum metallizer and is generally comprised of aluminum, zinc or alloys thereof.